HIGMEM and Memory Zones from newbie perspective

HIGHMEM

Earlier Linux kernels(about 10 years ago) where unable to support more than 1GB of  physical memory in 32 bit x86 systems. This was attributed to the way in which the VM was arranged. In earlier Linux 32bit systems with 3:1 split, the first 3GB of the VM was the user address space and the remaining 1GB was kernel address space. The 1GB allocated to the kernel can be mapped to  any part of physical memory. If the Kernel has to access any pages in physical memory then that has to be mapped to the its address space. Earlier kernel used static mapping and hence with 1GB address space, the kernel can only map 1GB of physical memory. Thus any physical memory beyond the size of 1GB can never be accessed by the kernel. Thus the earlier Linux kernels where restricted to using 1GB of physical memory in 32bit x86 systems.

The solution that was devised to solve this problem was high memory or simply HIGHMEM. In this strategy we divide the physical memory into high memory and low memory. The low memory still uses the permanent mapping. That is certain pages in the physical memory is always mapped to the kernel address space and that is called the low memory. The remaining part of kernel address space makes use of temporary mappings and that is called the high memory. That is the pages are mapped as when required. This enables the kernel to access any pages within 4GB range. The kernel data structures like linked lists must live in the low memory itself. This helps in maintaining pointer consistency. 

MEMORY ZONES

In Linux kernel the page frames are grouped logically in to three.They are
1.ZONE_DMA
2.ZONE_NORMAL
3.ZONE_HIGHMEM
There are some reasons in the Linux kernel to divide the physical memory as explained next.In some architectures there is a constraint on which a range of memory can be used. An example can be given in x86 architecture. In x86 architecture , the ISA devices are only capable of addressing the first 16MB of RAM. It implies that the for DMA operations, the ISA devices can only use the first 16MB of RAM. Some architectures does not have this constraint(Example :- PPC ). Besides this we have to accommodate the HIGHMEM solutions. With the division of memory, managing the memory becomes easy with each zone having a struct to store its details.
ZONE_DMA:

Above I mentioned that ISA devices in x86 has a constraint in memory addressing. So in x86 architecture ZONE_DMA is the group of pages that belongs to first 16MB of RAM. Since PPC does not have this constraint and hence in PPC ZONE_DMA is empty.

ZONE_NORMAL:

In x86 architecture , the first 896MB of page frames are permanently mapped to the Kernel Address space. Which can be other wise stated as , the first 896MB of Kernel address space is mapped to the first 896MB of physical memory.So that leaves 128MB of unmapped addresses in the 1GB kernel address space. This unmapped addresses can be used to make the temporary mappings from the high memory. The union of ZONE_DMA pages and ZONE_NORMAL pages gives us the low memory pages. If ZONE_NORMAL pages are not available for allocation, as a last resort, kernel can use ZONE_DMA but not vice versa.

ZONE_HIGHMEM:

ZONE_HIGHMEM is the collection of high memory pages. So in 32bit x86 that will be any memory above 896MB. They are not permanently mapped to the kernel address space as explained in HIGHMEM section.

Sources :

[1] Linux Device Drivers,3rd ed.,Corbet et al
[2]Linux Kernel Development by Robert Love 
[3] http://lwn.net/Articles/75174/